Diesel is an essential power fuel for engine and machine. In China, diesel frequently encounters supply crisis due to large market demand and serious lacking of petroleum. At the same time, consumption of diesel not only leads to serious environmental pollution such as haze, but also makes a threat to the health of human beings. A green diesel additive could significantly improve the combustion efficiency of diesel, and decrease exhaust (such as particular matter, NOx and CO etc) emission. Therefore, research and development of green diesel additive is of great importance.
Polyoxymethylene dimethyl ethers have received global attention as a potential green diesel additive. Polyoxymethylene dimethyl ethers refer to a series of homologue compounds CH3O(CH2O)nCH3, denoted by PODEn. PODE3-5 showed oxygen content (˜50%) and cetane number (70˜100). The fuel properties of PODE3-5 are close to that of diesel. Addition of PODE3-5 into diesel could significantly improve the combustion efficiency of diesel, and decrease exhaust (such as particular matter, NOx and CO etc) emission, thus upgrading the environmental and economic benefits.
Recently, many companies and research institutes are developing the production process of polyoxymethylene dimethyl ethers. Nevertheless, there has been no industrialized plant yet.
U.S. Pat. No. 5,959,156A, U.S. Pat. No. 6,160,174A, U.S. Pat. No. 6,160,186A, U.S. Pat. No. 6,392,102B1 by British Petroleum (BP) describe a process in which methanol or dimethyl ether is converted to formaldehyde via oxidative dehydrogenation, and then formaldehyde reacts with methanol or dimethyl ether forming dimethoxymethane and polyoxymethylene dimethyl ethers. The process is very complex, comprising unit operations including oxidative dehydrogenation, adsorption cooling, catalytic distillation, neutralization and separation. The selectivity to PODEn>1 in polyoxymethylene dimethyl ethers is less than 10%.
U.S. Pat. No. 7,700,809B2, US 20070260094A1, U.S. Pat. No. 7,671,240B2 by BASF describe the preparation of polyoxymethylene dimethyl ethers from dimethoxymethane and trioxane in the presence of acidic catalyst. The selectivity to PODE3-5 in polyoxymethylene dimethyl ethers is about 20 wt %, owing to the low water content in system (<1%). However, the cost of highly purified troxane and dimethoxymethane is too high. Besides, a considerable amount of by-products PODEn>5 are produced, thus complicating the separation process.
U.S. Pat. No. 0,056,830A1, U.S. Pat. No. 7,560,599B2 by Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, describe the preparation of polyoxymethylene dimethyl ethers from methanol and trioxane in the presence of acidic ionic liquid. The conversion of trioxane could reach 90%. However, ionic liquid is unfavorable due to high cost, difficult separation and recycling, thus complicating the process.
In production process of polyoxymethylene dimethyl ethers, reactor is the most important operation unit. Design of reactor would determine conversion of formaldehyde, selectivity to target products, production continuity and processing capacity. The published patents mainly focus on the technological process and catalyst, but rarely on the reactor design. Besides, feedstock cost is too high when using trioxane as source of formaldehyde, which seriously affect the economic benefits.
In bench-scale studies on preparation of polyoxymethylene dimethyl ethers, batch reactors are mostly used. It is unfavorable to be directly scaled-up due to low processing capacity and long production period. CN102249869A describes a cannula reactor for preparation of polyoxymethylene dimethyl ethers from methanol and trioxane in the presence of ionic liquid. However, this reactor has disadvantages including unstable system temperature, undermixing of reactants, nonuniform dispersion of catalyst, low conversion of reactants, high cost of trioxane and ionic liquid, and low stability of ionic liquid. CN102701923A describe a cannula reactor equipped with recirculated cooler, which aims at improving controlling of reactor temperature, but still fails to overcome drawbacks including undermixing of reactants, nonuniform dispersion of catalyst, low conversion of reactants, high cost of trioxane and ionic liquid, and low stability of ionic liquid.
In conclusion, there is need for developing a new reactor for industrialized production of polyoxymethylene dimethyl ethers and relevant processes. Therefore, development of a reactor with low liquid back mixing, high conversion of reactants, high heat exchange capacity and high reaction efficiency is of great importance for industry application.